Typical aircraft turbofan jet engines include an engine core, a nacelle that surrounds the engine core, and a fan that draws in a flow of air that is split into bypass airflow and engine core airflow. The nacelle provides a bypass duct that surrounds the engine core. The bypass airflow is transported through the bypass duct. The nacelle is configured to promote laminar flow of air through the bypass duct. The engine core includes a multi-stage compressor to compress the engine core airflow, a combustor to add thermal energy to the compressed engine core airflow, and a turbine section downstream of the combustor to produce mechanical power from the engine core airflow. The typical turbine section has two and sometimes three turbine stages. The turbine stages are used to drive the compressor and the fan. After exiting from the turbine section, the engine core airflow exits through an exhaust nozzle at the aft end of the engine.
In a turbofan engine, the fan typically produces a majority of the thrust produced by the engine. The bypass airflow can be used to produce reverse thrust typically used during landing. Thrust reversers mounted in the nacelle selectively reverse the direction of the bypass airflow to generate reverse thrust. During normal engine operation, the bypass airflow may or may not be mixed with the exhausted engine core airflow prior to exiting the engine assembly.
Several turbofan engine parameters have a significant impact upon engine performance. Bypass ratio (BPR) is the ratio of the bypass airflow rate to the engine core airflow rate. A high BPR engine (e.g., BPR of 5 or more) typically has better specific fuel consumption (SFC) and is typically quieter than a low BPR engine of equal thrust. In general, a higher BPR results in lower average exhaust velocities and less jet noise at a specific thrust. A turbofan engine's performance is also affected by the engine's fan pressure ratio (FPR). FPR is the ratio of the air pressure at the engine's fan nozzle exit to the pressure of the air entering the fan. A lower FPR results in lower exhaust velocity and higher propulsive efficiency. Reducing an engine's FPR can reach a practical limit, however, as a low FPR may not generate sufficient thrust and may cause engine fan stall, blade flutter, and/or compressor surge under certain operating conditions.
One approach for optimizing the performance of an engine over various flight conditions involves varying the fan nozzle exit area. By selectively varying the fan nozzle's exit area, an engine's bypass flow characteristics can be adjusted to better match a particular flight condition, for example, by optimizing the FPR relative to the particular thrust level being employed. Variable area fan nozzle (VAFN) systems, however, typically include multiple components that are selectively repositioned relative to the nacelle via one or more actuation systems.
To satisfy operational, safety, and certification requirements (e.g., Federal Aviation Administration requirements and European Aviation Safety Agency requirements), a VAFN system must satisfy structural damage tolerance and system reliability requirements. To satisfy system reliability requirements it may be necessary to monitor the VAFN system to detect, for example, actuation system failures that may result in degraded aircraft performance such as increased drag and/or decreased engine performance. Such monitoring, however, should be sufficiently reliable, which may be difficult to achieve with VAFN systems having multiple components that are selectively repositioned relative to the nacelle via one or more actuation systems.
Accordingly, VAFN systems that employ reliable monitoring are desirable, especially where the monitoring is accomplished in a simple and cost effective manner.